Séminaire CETHIL - Supersonic ejectors: from 0D models to Large Eddy Simulations

Prof. Sébastien PONCET, Université de Sherbrooke, Canada

Résumé au format pdf

Supersonic ejectors, also known as jet pumps, may be used as entrainment, mixing, and compression components or expanders, depending on the targeted applications (refrigeration, air-conditioning, desalination, compressed gas energy storage, among other examples). Recently, they have gained a renewed interest from the scientific community. Though their relatively low intrinsic efficiency, these are indeed simple devices without moving part able to increase the performance of the system at no additional cost. Single-phase ejectors are mainly integrated in heat driven refrigeration cycle for compression, while transcritical CO2 ejectors enable to recover part of the expansion work.
This seminar gives an overview of the numerical challenges encountered with ejector refrigeration systems that have been successfully addressed during the past six years at the LMFTEUS laboratory of Université de Sherbrooke. Various numerical approaches, namely thermodynamic models, artificial neural networks and turbulence models, have been developed and favorably compared to experimental measurements performed at our partners. These complementary models enable to highlight the preponderant geometrical parameters affecting the ejector performance and the main sources of exergy losses within the ejector. From those results, the injection of refrigerant droplets at different locations has been considered as a promising way to improve their performance. Results show also that the drop-in replacement of high GWP refrigerants like HFC by natural ones does not alter significantly the performance of the refrigeration cycle. Low-Reynolds number RANS and Large Eddy Simulations have also been performed to better understand the flow dynamics and exergy transfer inside the ejector. Results validate the extension of the compound choking theory to real gases and explain the entrainment limitation in supersonic ejectors. Exergy transport tubes provide a novel insight into the quantification and location of the transfers and the irreversibilities within the ejector.
The seminar will end with the perspectives on both the numerical modeling of ejectors and the ejector control strategies, which are of prime importance when capacity modulation is required.